Is spectrum twilight sky? (yes or no) If yes, earth-sun direction is used
to compute the velocity corrections. The BCV and HCV will be close but not
identical because the direction to the center of the sun's light is not the
same as the direction to the solar system barycenter.

Subtract solar gravitational redshift from twilight sky (yes or no)
If subgrav=yes and specsky=yes, subtract a correction for the sun's
gravity (0.636) from the barycentric and heliocentric corrections
computed by the program. If savebcv=yes, BCV, HCV, and SGRV are
written to the image header. HCV, the velocity of the sun relative
to the spectrograph with the gravitational correction subtracted,
should be used as the spectrum correction

Observatory name. If "file", read from image header using the following
keywords. Otherwise, look it up in IRAF's observatory database. If the string
is not found there, use the position given by the parameters below.